High-resolution optical imaging systems

Abstract

High-resolution imaging systems are provided. In one embodiment, an imaging system based on a Cassegrain or Schmidt-Cassegrain objective, with coaxial primary and secondary mirrors, is provided with a microdisplacement mechanism acting on the secondary mirror to displace the image on a focusing array. In another embodiment, two co-axial Cassegrain-type objectives are provided one within the other with a common focal plane array, which therefore detects combined wide field-of-view and narrow field-of-view images.

1. An optical imaging system for obtaining a high-resolution image of an observation area from a plurality of low-resolution images, each of said low-resolution images covering the entire observation area, said system comprising:
 a Cassegrain-type objective for forming said low-resolution images, said objective comprising a primary mirror defining a concave optical surface and a secondary mirror disposed coaxially with said primary mirror, said secondary mirror having a mirror plane and defining a convex optical surface therealong; 
 a focal plane array generally disposed at a focal plane of said Cassegrain-type objective for detecting said low-resolution images therefrom; 
 a microdisplacement mechanism for pivoting as a whole said secondary mirror about first and second orthogonal axes in said mirror plane to displace in a leveraged manner said low-resolution images as a whole between different positions along the focal plane array according to an acquisition sequence, for detection at said different positions; and 
 imaging means connected to said focal plane array for detecting the low-resolution images at each of said different positions and combining said low-resolution images according to a frame generation sequence matching said acquisition sequence to obtain said high-resolution image. 
 
   
   
     2. The optical imaging system according to  claim 1 , wherein said microdisplacement mechanism comprises first and second actuating means each exerting a force on said secondary mirror for pivoting said mirror about said first and second orthogonal axes, respectively. 
   
   
     3. The optical imaging system according to  claim 2 , wherein said forces exerted by the first and second actuating means are directed generally transversally to the mirror plane of the secondary mirror. 
   
   
     4. The optical imaging system according to  claim 3 , wherein said first and second actuating means operate said pivoting of the secondary mirror at a non-resonant frequency. 
   
   
     5. The optical system according to  claim 3 , wherein each of said first and second actuating means comprises a spring-driven actuator operating in a fully compressed mode. 
   
   
     6. The optical imaging system according to  claim 5 , wherein each of said spring-driven actuators comprises a piezoelectric element. 
   
   
     7. The optical imaging system according to  claim 1 , wherein said Cassegrain-type objective further comprises at least one correction refractive element for correcting aberrations in said low-resolution images. 
   
   
     8. The optical imaging system according to  claim 7 , wherein said at least one correction refractive element comprises a corrector element disposed in a path of light entering said Cassegrain-type objective. 
   
   
     9. The optical imaging system according to  claim 7 , wherein said at least one correction refractive element comprises a corrector element disposed between said secondary mirror and said focal plane array. 
   
   
     10. The optical imaging system according to  claim 1 , wherein said focal plane array is positioned at an apex of said primary mirror. 
   
   
     11. The optical imaging system according to  claim 1 , wherein said focal plane array is positioned beyond an apex of said primary mirror, and said primary mirror is provided with an opening extending therethrough at said apex thereof. 
   
   
     12. The optical imaging system according to  claim 1 , wherein said Cassegrain-type objective comprises a housing structure supporting said primary and secondary mirrors. 
   
   
     13. The optical imaging system according to  claim 12 , wherein said housing structure, primary mirror and secondary mirror are all made of a same material, thereby forming an athermalized ensemble. 
   
   
     14. The optical imaging system according to  claim 13 , wherein said microdisplacement mechanism comprises athermalized first and second actuating means each exerting a force on said secondary mirror for pivoting said mirror about said first and second orthogonal axes, respectively. 
   
   
     15. The optical imaging system according to  claim 14 , wherein each of said first and second athermalized actuating means comprises:
 an actuator exerting a force on said secondary mirror directed generally transversally to the mirror plane, said actuator being subject to temperature induced length variations; and 
 an athermalizing structure compensating for an effect of said temperature induced length variations on the pivoting of the secondary mirror. 
 
   
   
     16. The optical imaging system according to  claim 15 , wherein, for each of said athermalized actuating means, the athermalizing structure comprises:
 a lever subject to temperature induced length variations opposite the temperature induced length variations of the actuator, said lever having a first end interconnected with said actuator and a second end interconnected with a generally temperature-invariant pivot. 
 
   
   
     17. The optical imaging system according to  claim 16 , wherein said actuator is a piezoelectric element, said lever is made of aluminum and said pivot is made of invar. 
   
   
     18. The optical imaging system according to  claim 1 , in combination with a complementary imaging system similar to said optical imaging system, said complementary imaging system being positioned coaxially to said optical imaging system rearwardly of the secondary mirror thereof and non-obstructively to light entering said optical imaging system. 
   
   
     19. The combination according to  claim 18 , further comprising a signal combiner for combining the high-resolution images obtained by said optical imaging system and said complementary imaging system.